1. Technical Field
The present invention relates to a night vision system for detecting objects at relatively low visible light levels. In particular, the invention concerns an active night vision system using short-pulse laser illumination and a gated camera for improved distance-based imaging.
2. Background of the Invention
Night vision systems are utilized to allow a user to see objects at relatively low visibility light levels. Night vision systems typically are classified as either passive night vision systems or active night vision systems. In known passive night vision systems used in automotive applications, mid-infrared cameras are used to image objects using the ambient infrared light emitted by the objects in the environment. Mid-infrared night vision systems have relatively few pixels and, accordingly, images formed using such cameras have low video resolution and a relatively narrow field of view. Known active night vision systems utilize a near-infrared (NIR) diode laser or a filtered incandescent light source to generate NIR light. The NIR light is subsequently reflected off objects in the environment and is received by a NIR-sensitive camera. The camera generates a video signal responsive to received light.
An improved active night vision system is disclosed in U.S. Pat. No. 6,429,429 entitled xe2x80x9cA Night Vision System Utilizing A Diode Laser Illumination Module And A Method Related Thereto,xe2x80x9d which is herein incorporated by reference. That application describes an active night vision system that uses a NIR diode laser to illuminate the region forward of the vehicle and a CCD camera to process and display images within the illuminated region. Because NIR light is invisible to the human eye, the laser light can be formed into a high beam pattern to illuminate potential hazards without blinding oncoming vehicle operators.
A drawback of active night vision systems, in general, is that large or highly reflective objects that are close to the vehicle reflect very bright signals which can saturate the imaging system or, at least, obscure or make it difficult to discern objects which are further away. This drawback is particularly bad for fog, where the signal from the fog immediately around the vehicle can make it impossible to see objects farther away. Thus, there is a need for a night vision system and method related thereto that mitigates or eliminates blinding of the vehicle""s night vision system by close or highly reflective objects to the detriment of imaging objects that are farther away.
The present invention provides a night vision system and method related thereto for detecting objects at relatively low visibility light levels and mitigating the saturation effects of nearby objects. The night vision system uses short-pulse laser illumination and a gated camera for improved distance-based imaging with the added capability of seeing through fog, smoke, and other obscurants. The invention provides an imaging technique that allows the brightness of the apparent illumination to be reduced for nearby objects while, at the same time, being increased for faraway objects.
The night vision system in accordance with one method of the present invention includes a light source and a camera. The method includes activating the light source as a sequence of light pulses wherein each light pulse is increasing in intensity for a predetermined number of pulses to form a pulse train. The camera is activated as a corresponding sequence of detection windows wherein each of the windows corresponds to one of the light pulses for receiving reflected light resulting from the corresponding light pulse. The light pulses and detection windows are configured such that a time delay between each corresponding light pulse and detection window is increasing throughout the pulse train. In this way, objects nearer the night vision system are illuminated with lower intensity light and/or captured with less camera gain than objects further away to provide more uniform imaging of all detected objects. The composite image thus presents all detected objects substantially uniformly on the display.
In another embodiment, the controller uses camera gain modulation to adjust the relative intensity of reflected light for image presentation. In this scenario, the camera gain is increased as the delay for the camera window is increased, and the laser pulse amplitude is maintained constant. Thus, nearby objects are detected with less gain than far-away objects. The resulting composite image displays all detected objects at substantially the same intensity.
In yet another embodiment, the laser pulse amplitude and the camera gain are maintained constant, but the number of camera detection windows for each delay time increases as the delay time increases. In this method, more pulses from faraway objects and fewer pulses from nearby objects contribute to the resulting composite image, which again displays all objects at substantially the same intensity.
The present invention is advantageous in that it allows close, highly reflective objects to be displayed with substantially the same as or even less apparent brightness than objects that are further away. Moreover, under viewing conditions such as heavy smoke or fog, the present invention allows the veiling glare from the obscurants that are near the vehicle to be suppressed in the composite image, which allows the system to xe2x80x9csee throughxe2x80x9d the obscurant. Other advantages and features of the invention will become apparent to one skilled in the art from the following detailed description and the accompanied drawings illustrating features of the invention by way of example.